1. Field of the Invention
Many types of intermetallic compounds are known for use as hydrogen storage materials. Of particular interest herein are hydrogen storage materials provided by alloys of zirconium, chromium, iron and optionally titanium which are characterized by C14 hexagonal crystal structure and ZrCr.sub.2 stoichiometry.
2. State of the Art
A material suitable for storage of hydrogen must satisfy many demanding criteria. In addition to large storage capacity for hydrogen, a hydride formed during hydrogen absorption by a material should have moderately low enthalpy characteristics; that is, the quantity of heat per mole of alloy required for formation of the hydride is preferably relatively low. Also, the hydrogen storage material should absorb and desorb hydrogen quickly, preferably at a pressure near one atmosphere and near room temperature, and the material should show a minimum of hysteresis effects during a hydrogen absorption/desorption cycle.
Intermetallic compounds which have received much attention for use as hydrogen storage materials are provided by derivatives of ZrCr.sub.2 compounds, characterized by hexagonal C14 crystal structure and AB.sub.2 stoichiometry. It is well known that the ZrCr.sub.2 system is capable of absorbing copious quantities of hydrogen, but that hydrides formed from the ZrCr.sub.2 system are too stable to be of practical significance. In search of improved ZrCr.sub.2 -based systems, alloys have been prepared which contain other elements substituted for all or a portion of the zirconium, but with the AB.sub.2 stoichiometry maintained in the new alloy. For example, in Shaltiel et al., J. Less. Comm. Metals, 53, 117-131 (1977), there are described changes in the properties of AB.sub.2 Laves-phase ZrCr.sub.2 -based compounds by substitution of chromium with a 3d transition metal in accordance with the empirical formula Zr(Co.sub.x M.sub.1-x).sub.2 and Zr(Fe.sub.x M.sub.1-x).sub.2 wherein M=V, Cr, Mn and x is between zero and one. Other studies of partial substitution of zirconium in ZrCr.sub.2 alloys with titanium to form the hydrides of Ti.sub.1-x Zr.sub.x Cr.sub.2 pseudo-binaries are described in Jacob et al., J. Less, Comm. Metals, 73, (369-376) (1980).